High-speed wavelength channel selectors have been demonstrated in various schemes by positioning optical switches, such as semiconductor optical amplifier gates, optical amplifiers, electro-optic single crystal switches, or tunable optical filters, between a pair of arrayed waveguide gratings for optical packet signal processing systems.
The conventional schemes use combinations of the optical switches of semiconductor optical amplifier gates or electro-optic single crystal switches with silica or semiconductor wavelength multiplexer(s) and demultiplexer(s). However, there has been no report on hybrid or monolithic integrated high-speed wavelength channel selectors using electro-optic switches made of non-crystalline materials, such as electro-optic polymer or glass materials, with the wavelength multiplexer(s) and demultiplexer(s).
The conventional high-speed wavelength channel selectors will be described in detail hereinafter.
Referring to FIG. 1, there is shown a previous known conceptual scheme of a general high-speed wavelength channel selector.
This scheme is based on a combination of a 1×N wavelength demultiplexer (DEMUX) 2, optical switches 7, and an N×1 wavelength multiplexer (MUX) 3, which are connected with internal optical waveguides 1b and located between input and output optical waveguides 1a and 1c. For a detailed description, there is provided a reference directed to Y. Yoshikuni et al., NTT Review, Vol. 10, No. 1, pp. 14-20 (1998).
FIG. 2A represents a conventional high-speed wavelength channel selector including semiconductor optical amplifiers (SOAs) and planar arrayed waveguide gratings.
In this scheme, semiconductor optical amplifier gates (SOAGS) 7a are used as optical switches, and silica arrayed waveguide grating type 1×N demultiplexer 2a and N×1 multiplexer 3a are used as a wavelength demultiplexer and a wavelength multiplexer, respectively. The SOAGs 7a have advantages of small volume and optical gain, but have technical limitations on achieving a high yield on device fabrication due to complicate and high cost device fabrication processes. Furthermore, current technologies use either hybrid integration of the SOAGs with the silica arrayed waveguide gratings, which require cost and time consuming device integration processes, or monolithic integration of the SOAGs with semiconductor arrayed waveguide gratings, which have poor waveguide characteristics and difficulties in device fabrication yet.
This conventional scheme has been introduced by H. Ishii et al., IEEE Photonics Technol. Lett., Vol. 11, No. 2, pp. 242-244 (1999), and by R. Kasahara et al., IEEE Photonics Technol. Lett., Vol. 12, No. 1, pp. 34-36 (2000).
Referring to FIG. 2B, there is illustrated a conventional high-speed wavelength channel selector including optical amplifiers and planar arrayed waveguide gratings.
In this scheme, optical amplifiers 7b are used as optical switches, and silica arrayed waveguide grating type 1×N demultiplexer 2a and N×1 multiplexer 3a are used as a wavelength demultiplexer and a wavelength multiplexer, respectively.
The optical amplifiers 7b can be one of semiconductor optical amplifiers, fiber amplifiers, or planar waveguide-type optical amplifiers. The semiconductor optical amplifiers are most commonly investigated to realize the high-speed wavelength channel selector schemes. This composition of the optical amplifiers and the planar arrayed waveguide gratings has been developed not only for high-speed wavelength channel selectors but also for optical power equalizers between channels. In order to provide a further detailed description for this scheme, there are provided references directed to M. Zirngibl et al., IEEE Photonics Technol. Lett., Vol. 6, No. 4, pp. 513-515 (1994), and M. Zirngibl and C. H. Joyner, Electronics Lett., Vol. 30, No. 9, pp. 700-701 (1994).
Referring to FIG. 2C, there is provided a conventional high-speed wavelength channel selector employing electro-optic switches 7c based on single-crystal materials instead of the semiconductor optical amplifier gates shown in FIG. 2A.
When using electro-optic switches such as LiNbO3 switches in this scheme, monolithic integration with a wavelength multiplexer and a wavelength demultiplexer is not easily realizable, but only hybrid integration is required. The hybrid integration has a disadvantage of difficult device fabrication. This scheme has been introduced by Y. Yoshikuni et al., NTT Review, Vol. 10, No. 1, pp. 14-20 (1998).
FIG. 2D shows a conventional high-speed wavelength channel selector including tunable filters (T.F.) 7d, wavelength (λ) converters 5, and planer arrayed waveguide gratings 2a and 3a. 
In this scheme, sets of the wavelength tunable filters 7dand the wavelength converters 5 are used as optical switches, and the planar arrayed waveguide grating type 1×N demultipIexer 2a and N×1 multiplexer 3a as those in FIG. 2A are used as a wavelength demultiplexer and a wavelength multiplexer, respectively.
This scheme is depicted in U.S. Pat. Nos. 5,170,273, 5,194,977, 5,694,499, and 5,889,600. This scheme employing the wavelength tunable filters 7d and the wavelength converters 5 has difficulties on high-speed channel selection and channel locking of the tunable filters, and has a problem in practical system applications due to a complicate geometry caused by using many wavelength converters.
In FIG. 3, there is illustrated a conventional high-speed wavelength channel selector using Mach-Zehnder type optical switches 4.
As shown in FIG. 3, the high-speed wavelength channel selector is based on matrix-type composition of Mach-Zehnder type optical switches 4. The detailed description for this scheme is provided by A. El Fatatry et al., Electronics Lett., Vol. 24, No. 6, pp. 339-340 (1988) and R. Nagase et al., Journal of Lightwave Technology, Vol. 12, No. 9, pp. 1631-1639 (1994).
This wavelength channel selector has a switching scheme to connect a desired channel selected from input optical channels to a desired output path rather than to select a specific wavelength channel from many input wavelength channels. This scheme is not good for high-speed switching because it requires many cascaded switch operations between input and output paths and a complicate control process to operate the switches.
Referring to FIG. 4, there is shown a conventional scheme of a light source integrated wavelength converter 5.
This light source integrated wavelength converter 5 performs wavelength conversion for output signals of a high-speed wavelength channel selector when it is located at an output port of the wavelength channel selector, and has a scheme of converting a wavelength λin of an inputted optical signal into a wavelength λout by combining the input optical signal with an optical beam outputted from an optical pump beam source 20 at a nonlinear optical medium 19. In order to convert the wavelength of the optical signal into a desired channel wavelength by using this wavelength converter, the optical pump beam source 20 should be constituted by an N channel wavelength selectable laser, which is explained herein below in detail with reference to conventional schemes illustrated in FIGS. 5 to 9.
The wavelength converter shown in FIG. 4 provides an optical signal having the wavelength λout varying depending on change of a wavelength of the optical pump beam coming out of the optical pump beam source 20, and illustrates merely a very elementary wavelength converter scheme which does not guarantee wavelength conversion of the optical signal into the ITU specified grid channel.
Referring to FIG. 5, there is provided a first exemplary scheme of a conventional N channel wavelength selectable laser which can be used as an optical pump beam source for the wavelength conversion scheme shown in FIG. 4.
The N channel wavelength selectable laser includes two N×N wavelength multiplexers 10a and 10b, N laser gain media 11 located on optical waveguides 1 connecting the two N×N wavelength multiplexers 10a and 10b, N pairs of optical switches 4a and 4b placed on outer waveguides of the two N×N wavelength multiplexers 10a and 10b, respectively, a highly reflective mirror 13 coated on one side end of the N channel wavelength selectable laser, and a partially reflective mirror 12 coated on the other side end of the N channel wavelength selectable laser.
This laser scheme was proposed by B. Glance et al., Journal of Lightwave Technology, Vol. 12, No. 6, pp. 957-961 (1994). This scheme has a disadvantage of requiring many waveguides containing optical gain media and many optical switches, and is realizable only with electro-luminescent materials like semiconductor compound materials as the optical gain media. Therefore, this scheme is not suitable for a laser with optical gain media requiring external optical pumping.
Referring to FIG. 6, there is illustrated a second exemplary scheme of a conventional N channel wavelength selectable laser which can be used as an optical pump beam source for the wavelength conversion scheme shown in FIG. 4.
This channel wavelength selectable laser scheme is composed of a 1×N wavelength demultiplexer 2, N laser gain media 14 located on N optical waveguides 1b of the 1×N wavelength demultiplexer 2, a highly reflective mirror 13 coated on one side containing an opposite end of the N laser gain media 14 with respect to the 1×N wavelength demultiplexer 2, and a partially reflective mirror 12 coated on a cross section of an output terminal of a single waveguide 1a of the 1×N wavelength demultiplexer 2. This scheme is reported by M. Zirngibl, IEEE Communications Magazine, pp. 39-41 (December 1998).
In FIG. 7, there is shown a third exemplary scheme of a conventional N channel wavelength selectable laser, which can be used as an optical pump beam source for the wavelength conversion scheme illustrated in FIG. 4.
The N channel wavelength selectable laser includes an N×N wavelength multiplexer 10, a highly reflective mirror 13 coated on a cross section of one side of N optical waveguides 1a of the N×N wavelength multiplexer 10, N laser gain media 14 located on N optical waveguides 1b of the other side of the 1×N wavelength multiplexer 10, and a partially reflective mirror 12 coated on output ends of the laser gain media 14.
This scheme is reported by M. Zirngibl et al., IEEE Photonics Technology Lett., Vol. 6, No. 4, pp. 516-51B (1994), and provides N separate channel laser outputs with selectable laser oscillation on each wavelength channel of N channels unlike one combined channel output of the scheme shown in FIG. 6.
Referring to FIG. 8, there is presented a fourth exemplary scheme of a conventional N channel wavelength selectable laser, which can be used as an optical pump beam source for the wavelength conversion scheme shown in FIG. 4.
This N channel wavelength selectable laser includes an N×N wavelength multiplexer 10, one set of N laser gain media 14a placed at one side of the N×N wavelength multiplexer 10 and connected with the N×N wavelength multiplexer 10 through N numbers of optical waveguides 1a, a highly reflective mirror 13 coated on the opposite ends of the N laser gain media 14awith respect to the waveguide connected ends, another set of N laser gain media 14b placed at the other side of the N×N wavelength multiplexer 10 and connected with the N×N wavelength multiplexer 10 through N numbers of optical waveguides 1b, and a partially reflective mirror 12 coated on the opposite ends of the N laser gain media 14b with respect to the waveguide connected ends.
This scheme is proposed by M. Zirngibl and C. H. Joyner, Electronics Lett., Vol. 30, No. 9, pp. 701-702 (1994).
In FIG. 9, there is provided an exemplary scheme of a conventional electro-luminescent multi-channel selectable laser which can be used as an optical pump beam source for the wavelength conversion scheme shown in FIG. 4.
This electro-luminescent multi-channel laser includes a series of semiconductor gain waveguides 16 having different lengths with a partially reflective mirror 12 coated on one ends of the waveguides, and a diffraction grating 15 of Rowland circle type, both of which are formed on a semiconductor wafer 17.
This scheme is reported by J. B. D. Soole et al., Electronics Lett., Vol. 28, No. 19, pp. 1805-1807 (19924).
As shown in FIGS. 5 to 9, the schemes of the conventional N channel wavelength selectable laser, each of which can be used as an optical pump beam source for the wavelength conversion scheme illustrated in FIG. 4, are not suitable for practical realization because the schemes require many optical waveguides composed of an optical gain medium especially when the optical gain medium only employs electro-luminescent materials such as compound semiconductors or photo-luminescent materials requiring optical pumping.
Moreover, the conventional schemes of the wavelength selectable laser using semiconductor optical amplifier switches for the optical pump beam source of the wavelength converter, which is then be used as one component in a high-speed wavelength selector for high-speed optical packet signal processing, have technical drawbacks such as complicate fabrication processing, high manufacturing cost and low product yield. Furthermore, the conventional schemes of the wavelength selectable laser are not easily realizable because of practically difficult integration properties when the schemes use single crystal electro-optic devices and problems for wavelength selection and locking to a specified wavelength when the schemes use wavelength tunable filters.